Normally, such deflection type ink jet printers are continuous jet printers, in which the ink jet runs continuously and drops not used for printing are caught by a gutter (and typically re-circulated to the ink supply). Such printers may be arranged either so that undeflected ink drops pass from the ink gun to the gutter, and drops are deflected out of the path leading to the gutter in order to be printed, or so that drops are deflected into the gutter and printing takes place with undeflected drops. In either case, the printer may be constructed to apply different levels of the deflection to different drops, so as to provide a range of printing positions.
One known type of deflection ink jet printer typically has only one ink jet nozzle, and the drops are deflected to a variety of possible printing positions. Such printers are typically used for printing information and indicia such as xe2x80x9csell-byxe2x80x9d dates, code numbers, bar codes and logos onto foodstuffs and packages (e.g. yoghurt pots, eggs, milk cartons etc), manufactured articles, packaging and other articles which are conveyed past the print head on a conveyor belt or other conveying mechanism. Devices of this type are described, for example, in U.S. Pat. No. 5,481,288 (and WO-A-89/03768), U.S. Pat. No. 5,126,752 (and EP-A-0424008), U.S. Pat. No. 5,434,609 (and EP-A-0487259) and U.S. patent application Ser. No. 940667 (and EP-A-0531156), all of which are incorporated herein by reference. In another type of deflection ink jet printer, a plurality of ink jet nozzles are arranged in a row, and typically undeflected drops from each nozzle are used for printing while deflected drops are caught by the gutter (either a common gutter for all jets or a plurality of gutters). This type of printer is normally used for printing graphics.
In a normal continuous jet deflection type ink jet printer the ink leaves the nozzle in an unbroken stream of ink and breaks into drops a short distance from the nozzle. The ink jet is modulated, typically by applying a vibration to it in accordance with a modulation drive signal, in order to ensure that it breaks into drops in a controlled manner and at a desired frequency. The length of time between the moments when successive drops break from the ink jet is known as the drop period. Normally the drop period is controlled by, and can be determined from, the frequency of the modulation drive signal. The phase position of the moments when successive drops break from the ink jet will be referred to as the drop separation phase.
An electrically conductive ink is used and the voltage of the ink at the nozzle is held constant. An electrode, known as the charge electrode, is provided adjacent the path of the ink jet at the point where it breaks into drops. A voltage on the charge electrode will induce an electric charge in the part of the ink jet which is close to the electrode, and when a drop separates from the ink jet some of this charge is trapped on the drop. A deflection electrode arrangement creates an electric field which acts on the charge trapped on the drop to deflect it from the direction in which the ink jet is travelling when it leaves the nozzle.
In normal practice, different levels of deflection are applied to different drops by providing different voltages to the charge electrode for different drops, and thereby capturing different quantities of charge on different drops. As. an alternative, it has been proposed (e.g. in U.S. Pat. No. 4,122,458) to provide different strengths of the electric field for different drops. Whatever aspect of the system is changed to apply different levels of deflection to different drops, the changes must be made with a correct phase relative to the drop separation phase so as to ensure that each drop is deflected correctly. Therefore it is necessary to conduct an operation, known as phasing, to discover the drop separation phase.
During phasing a special signal is applied to the charge electrode. The frequency of this special signal corresponds to the drop period and its waveform is chosen so that the quantity of charge trapped on the ink drops depends on the phase position of the special signal relative to the drop separation phase. Normally the special signal is applied at several different phase angles during a phasing operation. By monitoring the level of charge trapped on the ink drops during phasing it is possible to identify the drop separation phase. The details of the phasing operation can vary greatly. U.S. Pat. No. 5,481,288 (and WO-A-89/03768) shows one approach. U.S. Pat. No. 3,761,941 shows a different approach.
The phasing operation depends on being able to detect the level of charge captured on the ink drops. One way of doing this is to provide an electrode, known as a phase sensor electrode, downstream of the charge electrode. The phase sensor electrode is very close to the path of the drops and a brief current signal is induced in it by each charged drop as it passes. It is optionally possible also to provide another electrode (known as a time of flight sensor electrode) further along the path of the ink drops, spaced by a known distance from the phase sensor electrode, which is also placed very close to the ink path and has a current signal induced in it by charged drops passing it. By measuring the time between signals induced on these two electrodes, it is possible to measure the ink jet velocity.
FIGS. 1 and 2 show plan and side views, respectively, of the main components of an example of an ink jet printer head using a phase sensor electrode and a time of flight sensor electrode. In FIGS. 1 and 2, the ink jet is emitted as a continuous stream from the nozzle 1 of an ink gun, and passes through a slot in a charge electrode 3. The continuous ink stream from the nozzle 1 breaks up into drops while it is in the slot in the charge electrode 3. The ink is electrically conductive and the ink gun is held at a fixed potential (usually zero volts for convenience and safety). The voltage on the charge electrode 3 induces a charge in the portion of the ink jet within the slot of the charge electrode, and as ink drops separate from the ink stream, the charge is captured in the drops. The amount of charge captured in each drop is controlled by varying the voltage applied to the charge electrode 3 (e.g. in the range 0 to 255 V). In this way, the charging signal applied to the charge electrode 3 controls the extent of the subsequent deflection of the ink drops.
The drops of ink then pass over the phase sensor electrode 5, which is used to detect the level of charge of the drops during a phasing operation as described above. The drops then pass between two deflection electrodes 7, 9, which are maintained at substantially different potentials (typically with a difference of 6 to 10 kV between them), so as to provide a strong electric field. This field deflects the charged ink drops, and the extent of deflection depends on the amount of charge on each drop. Drops with zero charge, or only a minimal charge, will pass through the field experiencing no deflection, or only minimal deflection, and will be caught by a gutter 11. Drops with higher levels of charge will be deflected sufficiently to miss the gutter 11 and will therefore continue in flight until they reach the surface 13 to be printed onto, and form a dot thereon. The range of possible deflection paths for dots to be printed ranges from the minimum degree of deflection necessary to miss the gutter 11 to the maximum amount of deflection possible before the deflected dot strikes the deflection electrode 7. The maximum and minimum deflected paths for printing are illustrated in FIG. 1.
Drops of ink having a minimal level of charge, so that the angle of deflection is not sufficient for the drop to escape the gutter 11, will pass over a time of flight sensor electrode 15 located between the deflection electrodes 7, 9 and the gutter 11. The time of flight sensor electrode 15 will respond to the charge on the drops to provide a signal which, together with the signal from the phase sensor electrode 5, can be used to measure the velocity of the ink drops as discussed above.
The phasing operation and time of flight measurement are carried out using a very low level of charge on the ink drops (normally of the opposite sign to the charge used for printing) so that the drops are still caught by the gutter 11. This limits the level of the signal which can be obtained from the phase sensor electrode 5 and the time of flight sensor electrode 15. In order to avoid these relatively small signals from being swamped by noise, the electrodes are configured as sensor electrode pins surrounded by and insulated from earthed shielding cylinders.
The arrangement illustrated in FIGS. 1 and 2 operates satisfactorily in practice but it has some drawbacks.
First, as is evident in FIGS. 1 and 2, both the phase sensor electrode 5 and the time of flight sensor electrode 15 occupy space in the line from the nozzle 1 to the gutter 11, and consequently the presence of these electrodes increases the path length of the ink drops from the nozzle 1 to the gutter 11. It is inherently desirable to minimise this distance, because the shorter the ink path length the less effect instabilities in the ink issuing from the nozzle have on the eventual position of ink drops, and also because the shorter this distance is the greater the clearance which can be provided between the end of the printhead and the surface 13 being printed onto for any given size of printed characters. It is not easy to reposition the sensor electrodes 5, 15 to reduce the path length, since the sensors must be positioned downstream of the charge electrode in order to detect charged ink drops and must be upstream of the gutter 11, and they must also be at a safe distance from the deflection electrodes 7, 9 in order to avoid arcing between the high voltages applied to the deflection electrodes 7, 9 and the sensors or their earthed shields.
Second, in order to detect the low level of charge on the drops used for phasing and time of flight measurement, the ink drops must pass very close (typically 0.35 mm to 0.45 mm) to the top of the phase sensor electrode 5 and the time of flight sensor electrode 15. This adds a further constraint to the alignment requirements when manufacturing the printhead, in addition to the requirement for the jet to be aligned correctly through the slot in the charge electrode 3 and with the gutter 11.
Third, the phase sensor electrode 5 tends to accumulate a layer of caked dried ink, mostly from splashes of mis-directed ink during start-up of the ink jet. Because the ink path passes very close to this sensor, only a small amount of caked dried ink can be tolerated on the sensor before it begins to interfere with ink drops passing along the correct path, and therefore the phase sensor electrode 5 must be cleaned frequently.
Fourth, if a splash of conductive ink hits the top of the phase sensor electrode 5 or the time of flight sensor electrode 15, the conductive nature of the ink tends to short the sensor electrode to the earth shield, preventing the sensor electrode from detecting any signal until the ink has dried and ceased to be conductive. This problem can be overcome by fitting an insulating cover over the top of the sensor electrodes 5, 15, but this increases manufacturing cost and also reduces the clearance between the electrode assembly and the ink jet.
In one aspect, the present invention provides a phase sensor electrode (and optionally also a time of flight sensor electrode) mounted on or combined with a deflection electrode. At least some embodiments avoid or reduce at least some of the drawbacks discussed above, but it is not an essential feature of the present invention to reduce all of them.
In one embodiment, the present invention provides a deflector plate for an ink jet printer comprising an electrically conductive deflection electrode, a layer of insulation on the side of the deflection electrode which would be towards the ink jet in use, and a sensor electrode or aerial overlying a part of the deflection electrode but separated from it by the insulating layer. In principle, it is possible to make this plate by using a self-supporting metal sheet as the deflection electrode, but is it preferred instead to use an insulating substrate to support the plate, for example made of a ceramic material, and then to lay down the deflection electrode, the insulating layer and the sensor electrode in turn on the substrate. This can be done, for example, by screen printing and baking according to known techniques for making hybrid circuit boards. In another aspect, the present invention includes a method of making an electrode plate for an ink jet printer comprising forming a deflection electrode, forming an insulating layer on it, and forming a sensor electrode on the insulating layer.
In another aspect, the present invention provides an ink jet printer having a deflection electrode and a sensor electrode or aerial in which the sensor electrode or aerial is formed on the deflection electrode but separated therefrom by an insulating layer.
In use, the deflection electrode is preferably maintained at substantially the same voltage as the sensor electrode, which will normally be the ground voltage of the sensing electronics to which the sensor electrode is connected. In this way, the sensor electrode does not substantially affect the deflection field caused by the deflection electrode. The potential applied to the other deflection electrode is then chosen to ensure that the desired deflection field is created. The deflection electrode on which the sensor electrode is mounted, and possibly the other deflection electrode also to some extent, shields the sensor electrode to minimise the amount of noise which the sensor electrode picks up.
Preferably, this arrangement is used to provide the phase sensor electrode. As discussed above, the presence of the time of flight sensor electrode is optional. If the time of flight electrode is required, then preferably it is also formed on a deflection electrode in this manner.
As will be appreciated from the discussion of the illustrated embodiments, at least some embodiments of the present invention allow the sensor electrode to be provided within the length of the deflection electrodes, so that no separate length of ink path is required to accommodate the sensor electrode. The sensor electrode as formed on the deflection electrode can be substantially larger than would normally be the case for the separate sensor electrodes of the type illustrated in FIGS. 1 and 2, and therefore the sensor electrode is more sensitive to the charged ink drops. Consequently, it can be mounted further away from the ink path, requiring less precise alignment of the ink jet and also permitting a greater build up of dried ink on the electrode before the accumulated dried ink interferes with the ink path. Preferably, the insulating layer extends beyond the edge of the sensor electrode to a substantial extent, and more preferably the entire surface of the deflection electrode on which the sensor electrode is mounted is covered by the insulating layer. Consequently, splashes of ink striking the sensor electrode or the deflection electrode tend not to bridge the insulating layer and short circuit the sensor electrode to the deflection electrode. It is also preferable that there is no insulation covering the sensor electrode, so that splashes of ink touching the sensor electrode are electrically connected to it. In this way, while the splashes are wet and still conductive, they act as extensions of the sensor electrode rather than acting as electrically separate covering layers which would tend to shield the sensor electrode and reduce its sensitivity.